Workers in the field of high-speed document processing, such as in the sorting of bank checks and like financial instruments, know that the art requires the use of machines and systems capable of moving and processing very large volumes of documents at rates of thousands of documents per minute, while performing multiple and interrelated operations upon each document as it travels through such machinery. Such operations might include, but are not limited to, printing upon the documents, reading data previously encoded thereon by a variety of processes, recording an archival image of the document by photographic or electronic-imaging techniques, and other processes and manipulations.
Workers understand that, while processing such large volumes of documents, it is vital that each individual document must be transported and processed singly, and that documents must remain in the order and sequence in which they were processed by the machine. To attain the rates of document processing required, the documents are fed and separated from each other by machinery, which is extensively designed and engineered to ensure that documents are fed one at a time with a very high degree of reliability. Should two or more documents be accidentally fed (a "double", or other "multiple") and processed together, extensive manual effort and time are required to track down the error created among the many thousands of documents which the machine must quickly process. For this reason, the most extreme measures are taken to ensure that the document feeding/separating stage always feeds documents one at a time, no matter what the condition of the documents.
There are still occasional unavoidable circumstances where the machinery will feed more than one document at a time. Examples might include documents which are stapled or glued together, documents which adhere because of ink or other surface treatments, or documents which are attached to one another by mutual tears or folds. Such cases are known in the art as "double-documents" or simply "doubles" or "multiples". The human operators of such sorting and processing machinery are aware that "doubles" are a costly and time-consuming event, and guard against them, the sheer volume of documents fed means that a "double" will occur in the machine from time to time.
For this reason, we find that machinery itself must contain a reliable device for detecting "doubles" as soon as possible after they have been fed, and preferably before any processing has been performed upon them. In this way, the operator may be warned of the presence of a "double" so it will cause minimal disruption to the normal flow of work. Such is a general object here.
Workers in the past have attempted many different techniques for reliably sensing and reporting a double-document. Such techniques must take account of the widely varying characteristics of the documents being fed (as regards thickness, density, opacity, etc.) as well as the increasing document transport speeds which are the result of continuing efforts to increase the processing rate. Techniques which have been tried in the past include:
--optical sensing: By shining a beam of light through the document and measuring how much of the beam passes through to impinge on a sensor, the additional thickness of a second document should produce a measurable change in signal.
This technique, while practical in principle, performs poorly in service. The wide range in characteristics of the documents being fed (especially as regards opacity and thickness) renders such a technique difficult to implement practically. Because such a system must tend to operate in a "fail-safe" mode, it seems to necessarily result in a high rate of "false reports" (of "doubles"). Such a false report is almost as disruptive as a real "double" would be.
Additionally, optical sensors are very susceptible to failure when encountering the high levels of dust and debris usually found in document processing machinery.
--Mechanical sensing: By passing the document between a known reference point and some moving effector, such as a stylus or roller, the thickness of the document may be measured, using various sensors. The additional thickness of a second document should be measurable.
Once again, the wide range of characteristics of the documents to be fed make this an impractical system. The thickest documents may well be more than twice the thickness of the thinnest, causing a high rate of "false double" signals. Additionally, the sensors required to detect mechanical variations of this order are sensitive and costly, and require skilled and time-consuming calibration to give a reliable result.
--Electronic sensing, relying on the variation of a parameter such as reluctance or permeability to detect the presence of documents.
Again, the range of document characteristics make these techniques less than successful; also they require the use of costly, custom-made sensing elements.
We have developed a technique which avoids the foregoin approaches, and gives high reliability, regardless of the nature or condition of the documents being fed; namely, a vacuum-separating and sensing system, and one which is apt for high-speed document processor/sorters. This invention seeks to teach improvements in such a system to enhance its reliability, serviceability and whole-life cost.